The present invention relates generally to the art of electrical rotating machines, such as generators or electric motors. More particularly, the invention relates to an arrangement for electrically insulating the shaft bearing of an electrical rotating machine.
The shaft of an electrical rotating machine is often supported by a bearing assembly maintained in position by the machine housing. In one known construction, the bearing assembly is supported by a portion of the housing known as the "end bell." As its name implies, the end bell is located at one axial end of the housing, and defines a hole through which the rotatable shaft freely extends. The machine may be constructed to either allow or prevent slight axial movement of the bearing assembly, depending upon the exigencies of a particular application.
Operation of the machine will often cause various currents to be induced in the machine housing. If allowed to flow into the bearing assembly, these induced currents may give rise to more frequent maintenance requirements. For example, the currents can arc to the bearing elements in a ball bearing or tapered roller bearing configuration, causing "pits" or other undesirable effects.
Efforts have been made to electrically insulate the bearing assembly from the machine housing. One prior art technique utilizes a ring for maintaining the bearing assembly about the shaft. The ring itself is inserted into a generally complementary receptacle defined in the end bell of the machine housing. A glass tape is provided between the radial outer surface of the ring and the radial inner surface of the receptacle to provide electrical insulation therebetween.
While this technique is effective at providing the desired electrical insulation, it gives rise to certain difficulties in manufacture. For example, a preferred approach is to machine the radial inner surface of the ring to accommodate the bearing assembly after the ring has been inserted into the receptacle of the end bell. To facilitate such machining, it is desirable that the ring remain fixed after insertion into the receptacle. This has been accomplished in the past by providing a very tight press fit between these two components.
To accomplish this press fit, the glass tape is first secured to the radial outer surface of the ring using an appropriate resin. After the resin has set, the glass tape and resin are machined to a diameter slightly larger than the diameter of the receptacle's radial inner surface. The end bell of the housing is then heated to produce a slight expansion. This expansion allows the ring to be slid into the desired position within the receptacle. After the end bell cools, the radial inner surface of the ring can be machined to a size for receipt of the bearing assembly.